The present invention relates to removal of metal ions from aqueous solution by electrolytic deposition. The process, in general, involves subjecting the aqueous solution containing trace quantities of metal ions to the action of direct current in the presence of at least one anode and a particulate cathode.
The typical cell employed consists of an anode separated from the particulate cathode by water permeable membrane. The particulate cathode is associated with a conductor which serves as a buss to complete the circuit.
In operation, the particulate cathodic material is maintained in a fluidized or a semi-fluidized state by the upper flow of the aqueous fluid to be treated through the cell. Apparatus employed for this purpose are disclosed, for instance, in U.S. Pat. Nos. 3,244,605 and 3,457,152.
The major deficiency of the operation is the nature of the materials used as the cathodic particles. There have been used particles of free metals, such as lead, tin, nickel and copper, as well as glass and plastic substrates, such as spheres coated, with the metals.
While the metals are, in general, passive to the aqueous solutions containing the metallic impurities in the pH range of from 2 to 12, they are attacked by the strong oxidizing acids at a pH of less than about 1.0 used to recover the deposited metals. As a consequence, recovery whether chemical or by anodic displacement results in either a complete or significant dissolution of the base metal which serves as the cathode. This loss is not only expensive, but can lead to the creation of toxic solutions as, for instance, where the cathode is lead.
In U.S. Pat. No. 3,766,036 issued to one of us, there is disclosed a process for the selective separation of ionic metallic species from aqueous solutions by metallic displacement. The active substance employed is a silicon metal alloy containing up to about 65% by weight silicon. In use, the alloy is selected to have a net electrochemical potential greater than a metallic species to be separated from the aqueous solution and less than the electrochemical potential of the metal species to remain in solution.
In the process when a metal contained in the alloy is exchanged with a metallic ion in solution, the alloy is continually consumed and there is no net change in the overall metallic ion content of the aqueous solution, although the nature of the ionic metal has changed.